VII] FUNCTIONS OF BARK 97 



~Ulmus campestris, Eobinia, Euonymus, Quercus Suber, 

 Acer campestre, Birch, Hazel, &c. the bark is easily 

 indented with the finger nail and may be termed soft. 



But in many cases the bark contains such abundance 

 of sclerenchyma or of fibrous elements that it is distinctly 

 hard, and when the hard elements are in excess as e.g. 

 in the rough bark at the base of old Birch-trees, and the 

 rough bark which developes on some Beech-trees it is 

 often termed stone-bark. 



The alternation of harder and softer layers is a fruitful 

 source of exfoliation e.g. Scots Pine, Birch, &c. These 

 layers are not periodic in the same sense as annual rings, 

 and they tend to form more rapidly in youth than in 

 age. 



The primary function of the tegumentary layers is no 

 doubt that of protection from loss of water by evaporation, 

 but one of the principal secondary functions of the tegu- 

 mentary layers is to protect the living tissues within 

 especially the very delicate cambium from the direct 

 rays of the sun, and probably as much from the so-called 

 heat rays as from the blue-violet rays so destructive to all 

 living protoplasm. 



In the case of the youngest twigs this sheltering action 

 is supplemented by the fan-like shading afforded by the 

 leaves, but later on the thick non-conducting periderm 

 layers and bark may have to be relied on. 



In accordance with this, it has been noticed that trees 

 which grow in exposed situations the so-called light- 

 demanding trees of forest lore have the cambium well 

 protected by thick bark, e.g. Oak, Eobinia, Scots Pine, 

 Larch, &c., or, as in the case of the Birch for instance, 

 with repeatedly laminated shells of cork of alternately 

 thin walled air-containing cells and ordinary cork cells. 

 It has even been suggested that the white colour, in part 



w. v. 7 



